A flow rate Q of a fluid that flows inside a passageway of a flow control valve can be derived by measuring the differential pressure of the fluid between the primary side (upstream side) passageway and the secondary side (downstream side) passageway, the valve opening degree, and the flow coefficient (Cv value), and then plugging those values into equation [1] below.Q=A·Cv·√ΔP   [1]
Therein, A is the cross sectional area of a throttle part of the valve, and ΔP is the pressure differential of the fluid between the upstream side and the downstream side.
The control valves disclosed in, for example, in Japanese Examined Patent No. 2772159, Japanese Unexamined Patent Application Publication No., S60-168974, Japanese Examined Patent Application No. H07-103945 (Patent Documents 1-3, respectively) are known examples of flow control valves that are capable of measuring the flow coefficient and the flow rate of a fluid that flows inside a passageway.
In an integrated process control valve disclosed by Patent Document 1, a first pressure sensor that measures an inlet fluid pressure P1 is disposed at a portion of a passageway on an inlet side of a valve body, and a second pressure sensor that measures a fluid pressure P0 is disposed at a portion of a passageway on an exit side; in addition, a controller derives a differential pressure ΔP between the fluid pressures P1, P0, reads the flow coefficient (Cv value) of the valve from a table, and calculates the flow rate Q by Q=kCv√ΔP/Gf, wherein k is a constant and Gf is the specific gravity of the fluid.
The flow control valve disclosed by Patent Document 2 comprises: a valve opening degree detecting means, which detects the valve opening degree; a first pressure detecting means, which detects the fluid pressure inside the upstream pipeline of the valve body; a second pressure detecting means, which detects the fluid pressure inside the downstream pipeline of the valve body; and an electrical means, which calculates, based on the electrical output signals of the first and second pressure detecting means and the valve opening degree detecting means, the fluid flow rate that flows inside the pipeline.
A butterfly valve, which comprises the flow rate measuring apparatus disclosed by Patent Document 3, is configured so that pressure outlet ports are formed at four locations on both the upstream side and the downstream side, and the differential pressure before and after the valve is measured by averaging the pressures sampled inside annular shaped cavities that extend in the circumferential directions and are formed on both the upstream and downstream sides of the inner circumference of a seat ring.
Incidentally, turbulent flow occurs before and after the throttle part of the valve, resulting in large pressure fluctuations; therefore, to measure the flow rate Q of the fluid with high accuracy, the upstream side fluid pressure outlet part and the downstream side fluid pressure outlet part must be provided at positions sufficiently spaced apart from the throttle part of the valve.
Furthermore, if the aim is to reduce the size of the flow control valve that is provided with a flow rate measuring means, then it is usually advantageous to dispose the flow rate measuring means inside an actuator that drives the valve element. Consequently, the control valve recited in the abovementioned Patent Document 1 features separate external piping that connects a controller part, which is provided with the flow rate measuring means, with an upstream side fluid pressure outlet part and a downstream side fluid pressure outlet part and thereby guides the fluid pressure to the controller.
Nevertheless, using piping to guide the fluid pressure from the upstream side fluid pressure outlet part and the downstream side fluid pressure outlet part to the controller in the control valves as disclosed by Patent Documents 1 and 2 poses some problems, including leakage of fluid from the joints of the piping that surround the valve body, an increase in the number of parts and the labor needed to assemble the piping, and a corresponding increase in cost.
In addition, the reduction in size of the butterfly valve recited in the abovementioned Patent Document 3 comes at the sacrifice of measurement accuracy. Namely, because the positions of the pressure outlet parts on the upstream and downstream sides are not sufficiently spaced apart from the valve element, the upstream side and downstream side pressures fluctuate greatly, leading to the problem wherein the flow rate cannot be measured with high accuracy.